Aux I/O is the new feature of Pro Tools 2022.9 which is hardest to explain but it’s going to be the gift that keeps giving in terms of opening up new ways of using Pro Tools. ARA 2 integration with Melodyne is going to be incredibly useful, but only for doing one thing. I don’t think we’ve even imagined all the ways Aux I/O is going to be useful! However it does take more than a sentence to explain what it is and what it does. This article will help us appreciate what Aux I/O can do and what limitations it has.

What Is Aux I/O?

Aux I/O allows you to add any Core Audio device as extra inputs and outputs to your current Playback Engine. It’s technical but it’s extremely powerful and addresses some persistent pain points which have dogged certain users for a long time.

Aux I/O is a Mac-only feature, available on Big Sur or later. This is because it relies on access to Core Audio. It allows you to have your primary Playback Engine set up to your preferred setting and still be able to stream to and from other Core Audio devices. This is very significant as it allows Pro Tools to access other hardware and applications other than the currently selected Playback Engine, something which required various workarounds to achieve in the past.

Pro Tools, unlike most DAWs, has a single setting for the Playback Engine which controls both the inputs and outputs. Pro Tools HDX users will be aware of how well the HDX/Hybrid audio engine works but also how it can be very awkward to incorporate other hardware or Core Audio devices with it. The choice has been to use HDX or run natively but if you wanted to combine Core Audio and HDX things got more complicated.

To see how this works watch this short intro video from Avid

Access to Aux I/O is from the I/O Setup window where a new button opens a window in which new paths can be set up which can access both hardware and virtual devices such as the Dolby Audio Bridge or Dante Virtual Soundcard. Aux I/Os can be given custom names in the I/O setup, one limitation to be aware of is that while hardware can act as both an input or an output, virtual devices have to be either an input or an output. You can roundtrip audio to and from software applications, it just requires setting up two separate core audio devices to avoid creation potential feedback loops.

Using The Pro Tools Audio Bridge To Route Between Apps

Presets are provided in the form of the Pro Tools Audio Bridge - Virtual Devices of various channel widths from 2 to 64, which are pre-installed and ready for use, but the advanced user can of course set up their own, though the sync performance using the pre-installed Pro Tools Audio Bridge is better than that which can be achieved using custom Aux I/O.

A good place to start would be to route audio from an alternative audio application into Pro Tools. For example it’s possible to route outputs from MainStage directly into Pro Tools and MIDI from Pro Tools into MainStage. If MIDI is routed from Pro Tools to MainStage via the IAC Driver, previously inaccessible Logic virtual instruments can be used within Pro Tools, or at least driven from MIDI in Pro Tools, with audio routed back into Pro Tools via Aux I/O. See how this works in practice in this short video.

Another group of users who will benefit from Aux I/O will be HDX users who will be able to access the Dolby Atmos renderer in software without having to change their Playback Engine from HDX. In this way they can keep using the HDX engine but have an alternative route out of Pro Tools to access their renderer.

The Dolby Audio Bridge is available in Pro Tools as an option in the Playback Engine settings but this does limit things. HDX users in particular can no longer access their systems in the same way. Using Aux I/O to provide additional I/O is more flexible. See how to use Aux I/O with the Dolby Atmos Renderer in the video below.

Using Aux I/O with Zoom And Other Video Conferencing Software

Involving fewer channels but every bit as useful are the possibilities for routing to or from online conferencing and collaboration tools like Zoom, Teams and the like. Unlike the MainStage example above, although only using two channels, using Zoom involves bidirectional audio and in this video from Avid the steps necessary to use Pro Tools and Zoom are clearly laid out.

Auditioning Sound Effects Libraries Using Aux I/O

In the past various workarounds were used to route system audio into Pro Tools systems. often this involved routing the built in outputs on a computer into a spare pair of Pro Tools inputs in the analogue domain using a cable simply because it was the most reliable method. Aux I/O opens up flexible possibilities, particularly for Post users who use third party solutions for managing their sound effects libraries, though this is just as useful for music mixers routing streaming services into Pro Tools for checking against reference tracks. See how simple it is in the video below.

Latency and Clocking With Aux I/O

Pro Tools users will no longer have to compromise their Playback Engine’s performance by creating Aggregate Audio Devices to incorporate additional hardware, though it should be said that there is no delay compensation on an Aux I/O and additional latency will be introduced on the Aux I/O, though in practice this is minimal, dependant on buffer settings. The Playback Engine offers reliable performance with delay compensation and, depending on the particular playback engine, low or negligible latency. Aux I/O doesn’t inherit these characteristics and should be seen as complementary to your Playback engine. Latency might vary between passes and it even has its own Ignore Errors During Playback Setting. Reported performance is apparently very good but it’s important to understand the difference between the Playback Engine I/O and Aux I/O.

Clocking and preventing drift can be achieved for interfaces using conventional clocking if available but for virtual devices this isn’t possible. The Pro Tools Audio Bridge virtual devices which come preinstalled have a built in clocking mechanism which custom virtual devices don’t have so these should be used where possible.

Aux I/O is one of those developments which will have a huge impact on the uses people put Pro Tools to. However we suspect that there are clever uses which haven’t yet been invented. As we said at the beginning of this article, Aux I/O is likely to be the gift which keeps giving…